1. Field of the Invention
The present invention relates to a metal vapor laser and more particularly to an ultraviolet metal vapor laser, such as a helium-cadmium laser, which uses a glass substrate which has a thickness in the range of 3.0 to 10.0 millimeters and which transmits at least 85% of light energy at a wavelength of 325 nanometers for the output coupling mirror assembly in order to simplify fabrication of the ultraviolet metal vapor laser and increase its lifetime.
2. Description of the Prior Art
Metal vapor lasers, such as helium-cadmium lasers, have been commercially available for a number of years. Some of these metal vapor lasers are capable of producing outputs of light energy in the deep blue visible region of the spectrum at 442 nanometers. These helium-cadmium lasers operating at the 442 nanometer laser emission line utilize a hard glass to metal sealed laser resonator internal to the plasma tube. Other of these metal vapor lasers are capable of producing outputs of light energy having a wavelength of 325 nanometers. These ultraviolet metal vapor lasers, such as helium-cadmium lasers, operating at the 325 nanometer laser emission line utilize plasma tubes which are sealed at the ends with fused silica or cyrstaline quartz brewster angle windows. Within the plasma tube is a mixture of two or more gases which serve as the optically active laser gain medium. When such mixture is exited by a direct current discharge, ultraviolet laser lines are excited. Laser mirrors situated at either end of the laser plasma tube, and external to the plasma tube were used in a resonant cavity to cause optical feedback of ultraviolet light amplified in the active gain region of the plasma tube. Schott K5 glass and Schott BK7 glass are used for visible light energy at a wavelength of 442 nanometers. Both glasses have the ability to be fritted at a temperature below 500 degrees Centigrade. Schott is the trademark of Schott Glaswerke, Mainz, West Germany. Schott K5 and Schott BK7 are codes which Schott Optical Glass Company, Durlyea, Pa. uses to designate glass.
Damage of intracavity optical elements, such as the brewster windows and damage of the laser mirror coating occur when exposed to a beam of ultraviolet light energy. The intracavity power is fifty to one hundred times the emitted ultraviolet power so that the intracavity optical elements are subjected to a substantial ultraviolet light energy density and are damaged thereby. The damaged optical elements cause loss of intracavity transmission. Higher losses within the laser mirror optical coating subsequently causes loss of emitted laser power. The influence of intracavity losses on the emitted power of a laser is very non-linear in that a very small intracavity optical loss will cause a very large loss in emitted output of the laser discharge plasma tube thereby reducing the lifetime of the ultraviolet metal vapor laser.
The most common ultraviolet transmission materials used for intracavity optical element or laser mirror substrates are crystaline quartz or synthetic fused silica. These materials have a very low thermal expansion coefficients and are not suitable for hard glass to metal sealing at low temperatures in the range of from 300 to 500 degrees Centigrade. It is important to have a hard glass to metal sealing temperature within this range because the common materials for reflecting multilayer dielectric coatings will be destroyed during the sealing process if sealing temperatures are above this range. A typical failure of reflecting multilayer dielectric coating during hard glass to metal sealing is "crazing" which is the cracking of the reflecting multilayer dielectric coating to thermal expansion mismatch of the dielectric stack with the substrate material.
It is desirable to improve the lifetime of ultraviolet metal vapor laser by eliminating both intracavuity optical elements which can be damaged by the intracavity laser beam and protecting the critical reflecting multilayer dielectric coating of the laser mirror from damage by the ultraviolet laser beam. It is also desirable to provide these improvements while providing good optical transmission of ultraviolet light energy outside the resonator cavity and at the same time providing a good hermetic vacuum sealing for the plasma tube which is impermeable to contaminant gases and vapors entering the plasma tube as well as to gases and vapors leaking from within the plasma tube.
U.S. Pat. No. 4,233,568, entitled Laser Tube Mirror Assembly, issued to Randolph W. Hamerdinger and Robert C. McQuilan on Nov. 11, 1980, teaches a laser tube assembly which includes a laser tube and pair of laser mirrors. The laser tube has a hard glass to metal sealed laser resonator which is internal to the plasma tube for use in a helium-cadmium laser. Each laser mirror is sealed to one end of the laser tube. The sealant is able to withstand the relatively high temperatures which are utilized to remove contaminants during fabrication thereof. The sealant is also able to minimize gas permeation therethrough during utilization of the laser tube.
U.S. Pat. No. 3,904,986, entitled Gas Laser Tube, issued to Karl Gerhard Hernqvist on Sept. 9, 1975, teaches a gas laser tube which includes an elongated envelope, an active laser medium, an output coupling mirror assembly which is disposed on one end of the elongated envelope and a reflector mirror assembly which is disposed on the other end thereof.
U.S. Pat. No. 4,149,779, entitled Internal Laser Mirror Alignment Fixture, issued to Randolph W. Hamerdinger on Apr. 17, 1979, teaches a laser tube mirror alignment fixture.
U.S. Pat. No. 4,224,579, entitled Metal Vapor Laser Discharge Tube, issued to Calvin J. Marlett, Edwin A. Reed, Richard C. Johnson and William F. Hug on Sept. 23, 1980, teaches a metal vapor laser which includes an envelope, a capillary tube, an anode, a cathode and a pair of mirrors. The metal vapor laser discharge tube also includes a reservoir of helium, an evaporator and a condenser. The evaporator is fluidly coupled to the capillary tube adjacent to the anode. An active material is placed in the evaporator. The condenser is fluidly coupled to the capillary tube adjacent to the cathode. The laser discharge tube further includes a heater. The heater is mechanically coupled to the evaporator. The heater applies heat to the active material in the evaporator in order to produce a vapor the positive ions of which the cathode draws to the condenser.